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Introduction
The evolution of the Antarctic notothenioid fishes in the cold waters of the Southern Ocean has resulted in a marked and shared intolerance to heat (Somero & DeVries 1967, Bilyk & DeVries 2011). Though most extant members of the suborder Notothenioidei inhabit the Southern Ocean, a few species are only found in the warmer waters around New Zealand and southern South America. Divided into two distinct groups, these include both the members of the three basal notothenioid families (Bovichtidae, Pseudaphritidae, Eleginopidae), which diverged prior to the origin of the cold adapted Antarctic notothenioid clade, and at least 16 species nested within the Antarctic notothenioids but now permanently found outside of Antarctic waters (Eastman 2005). Unlike the basal species, the ancestors of this latter group of secondarily temperate species presumably once shared the reduced heat tolerance noted among the endemic Antarctic species.
The shallow coastal waters of southern New Zealand are home to one such secondarily temperate species, the black cod (Notothenia angustata Hutton). Despite a residence time in cold temperate waters that may reach back 11 m.y.a., N. angustata continues to exhibit a number of cold adaptations characteristic of the endemic Antarctic species. Although they inhabit continually ice-free waters, this species retains some functional genes for antifreeze glycoprotein (AFGP) not found among the basal notothenioid families (Cheng et al. 2003). At their environmental water temperatures, they also show intermediate levels of both cellular membrane lipid saturation (Logue et al. 2000) and ubiquitin conjugated proteins (Todgham et al. 2007) between cold temperate and endemic Antarctic species. However, whether whole organism traits, like heat tolerance, show a similar pattern remains unknown.
Heat tolerance in fishes has typically been determined using two methodologies, either the measurement of their upper incipient lethal temperature (UILT), or their critical thermal maximum (CTMax) (Kilgour & McCauley 1986). The UILT, based on lethal dosage methodologies, measures tolerance as the temperature at which median mortality first becomes independent of the length of exposure, typically determined from exposure times ranging from a day to a week (Fry 1947, Cossins & Bowler 1987). The first study of heat tolerance in the Antarctic notothenioids used this methodology and found UILTs between 5 and 7°C in three...